Instead of elaborating schema to encode entity mentions with complex structures, another direction based on sequence tagging techniques is to employ multiple sequence tagging models or layers, that are arranged in a series. [1]} employ several sequence tagging models, each of which is used to recognise entity mentions belonging to a group of several entity categories without nested structures. Similarly, [2]} stack several BiLSTM-CRF layers together, each of which is used to recognise entity mentions belonging to a particular nesting layer. Note that, although these two methods achieve decent results in nested NER benchmarks, they have some difficulties in dealing with special nested structures. The cascade approach proposed by [1]} cannot deal with nested mentions of the same entity category. For example, one DNA mention might contain another DNA mention. The layered approach proposed by [2]} cannot deal with multi-type entity mentions. For example, one mention might be annotated as both Protein and Drug/Compound.
